i think that's an excellent question. since my area of expertise is chemistry and not climatology, i can only speculate that it's a combination of peer pressure, the desire to get funding and their allowing governments to manipulate their findings.  there have been several instances in which members of the ipcc have quit amidst claims that the final reports issued did not reflect their research. since the final ipcc reports are reviewed by politicians prior to being published, i'm not surprised with the claims made. here's a relevent story. 
Label: Non-Sarcastic